1. Field of the Invention
This invention relates to an auxiliary brake system, and more particularly, to an auxiliary brake system which enhances the braking ability of a compression-releasing engine brake unit by effectively utilizing a turbocharger having a supercharge-degree adjusting function.
2. Description of Related Art
Large-sized vehicles such as trucks and buses, which are heavy in weight and which produce a large inertial force during running, require a large braking force at the time of braking. In addition to an ordinary brake unit such as drum brake unit and disk brake unit, large-sized vehicles are generally equipped with an auxiliary brake system such as engine brake unit and exhaust brake unit.
A compression-releasing engine brake unit attached to an internal combustion engine operates to permit a piston to compress suction air, sucked into a combustion chamber of the engine, in a condition that fuel supply is interrupted, and to cause an exhaust valve to open to discharge the compressed air when the piston reaches the vicinity of top dead center in a compression stroke, whereby the compression work by the piston is absorbed to generate a braking force.
The braking ability of an engine brake unit of this type increases with the increase in an amount of suction air or the volumetric efficiency (charging efficiency). In this regard, it is considered that the suction air pressure (boost pressure) at the time when the engine brake unit is in operation may be increased by use of a turbocharger provided in an internal combustion engine and having a turbine and a compressor.
An engine brake unit disclosed in Japanese Unexamined Utility Model Publication No. 61-33933 utilizes a variable geometric turbocharger (hereinafter referred to as VG turbocharger) in which an inclination angle of nozzle vanes, disposed in a turbine nozzle section around turbine blades of the turbocharger, is variable in accordance with engine rotation speed. When the engine brake unit is in operation, the inclination angle of the nozzle vanes is decreased to reduce a nozzle opening area, thereby increasing an exhaust flow speed. As a result, the turbine and the compressor can rotate at high speeds, so that the charging efficiency (supercharging degree) increases, thereby increasing the engine brake force.
The VG turbocharger operates, while adjusting the supercharging degree, not only when the engine brake unit is in operation, but also when the engine operates in an ordinary state (especially, in an accelerated operation state) in which fuel is supplied while rendering the engine brake unit inoperative. The exhaust gas, discharged from a combustion chamber when the engine is driven and supplied with fuel, contains a combustion gas produced by combustion of fuel. Thus, an exhaust flow amount and exhaust pressure become quite large, so that the turbine rotation speed increases. From the viewpoint of preventing excessive rotation of the turbine, it is inappropriate to extensively decrease a nozzle opening area (nozzle-vane opening) for adjustment of the supercharging degree. That is, there is a restriction in decreasing the level of a low limit of the nozzle opening area. Although not specifically mentioned in the aforementioned U.M. publication, it is understood that the nozzle opening area at the time when the engine brake unit is in operation is adjusted to a value falling within a limited range which is determined taking into account the above restriction. In this case, a variable range within which the nozzle opening area or the supercharging degree varies during the operation of the engine brake unit is equivalent to the variable range employed at the time the engine brake unit is disabled.
When the engine brake unit is in operation, the compressed suction air is discharged without being subjected to combustion expansion or without a volumetric change. Thus, the supercharging degree and the turbine rotation speed do not increase, if the nozzle opening area varies within the limited range conforming to the case where the engine brake unit is disabled. Insufficient supercharging degree sometimes causes a problem of inadequate engine-braking-force enhancing effect of the turbocharger.